So far mainly radio frequency (RF) links are used in order to provide for a communication between a ground station and a terminal carried by a satellite in deep space. In such communications systems typically one terminal is attached to the satellite whereas another terminal is either a ground based terminal or a terminal carried aboard of a second satellite in an orbit closer to the earth.
Various researchers and developers, however, are currently developing optical links to support deep-space communication to satellites.
The longer the distance between the two communication terminals gets, the more careful the link budget has to be designed. In particular the downlink budget, that is the budget of the link between the satellite in deep space and a ground based terminal, or a terminal in a lower orbit, has to be designed with great care.
But there are many other situations where optical links are needed that are able to reliably carry high data rates even if the optical channel is obstructed by fog or clouds, for instance.